Non-linear diffusion during phase separation and symplectite formation in alkali feldspar: an experimental and modelling approach (TP8)

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In the precursor project we studied microstructure evolution during spinodal decomposition of alkali feldspar in experiment and theoretically. With the successor project we want to extend this study by (i) addressing nucleation-growth as an alternative reaction pathway for exsolution and (ii) investigating in detail Na+/K+ interdiffusion in alkali feldspar. (i) To induce exsolution by nucleation and growth, isothermal anneals will be done at conditions where the homogeneous alkali feldspar is metastable. Experiments will be done at high pressure in the 1 to 2 GPa range, where the metastable region in the alkali feldspar binary extends to high temperature, and the overall reaction kinetics is fast. The exsolution microstructure and microchemistry will be investigated using electron beam microanalytical techniques with high spatial resolution including FEG-SEM, FEG-EMP and TEM combined with FIB sample preparation. The numerical model that was developed in the precursor project will be complemented with a nucleation module to extract process parameters from comparing modelled and observed microstructures and phase compositions. (ii) From experiments in the precursor project we find that the geometries of concentration profiles produced from Na+/K+ exchange between alkali feldspar and NaCl-KCl salt melt deviate from what is expected for simple interdiffusion. Exchange fronts propagating through the crystal show a sharpness, which depends on the composition difference between the pristine and the exchanged domains of the grain but does not change with time. We hypothesize that this may be due to strain occurring at the interface between exchanged and pristine crystal due to lattice mismatch (coherence-strain). The mechanisms by which this occurs are, however, elusive. Solving the question of coherence strain effects on cation interdiffusion is fundamental for the understanding of interdiffusion and exsolution in mineral systems. We suggest doing cation exchange experiments using different starting materials including isotopically labelled KCl with well-defined geometry. The microchemistry, defect density and lattice distortion at the exchange fronts will be investigated using high resolution analytical techniques, and conceptual models for the influence of coherency strain on Na+/K+ interdiffusion in alkali feldspar will be developed and parmeterized.